This invention relates to electrical circuits for converting a variable input voltage to a corresponding proportional variable current, and for establishing the voltage level of the supplied current.
In the conversion of electrical signals representing digital information into analog electrical signals, it is customary to use ladder networks of resistances to establish the required binary weighting between the various currents corresponding to the input digital signals. Biasing current to the transistor controlled resistance ladder network is commonly provided by a voltage-to-current conversion circuit using an operational amplifier which creates a current in the ladder which is directly proportional to that of a voltage reference source. A typical prior art circuit for biasing a digital-to-analog resistance ladder is shown in FIG. 2 of the present drawings and will be discussed further below.
Disadvantages of such prior art circuits employing operational amplifiers are the relatively large number of devices required to implement the operational amplifier, and the problems arising from the associated compensation circuitry. Generally a compensation capacitor in the neighborhood of 20 to 80 picofarads is required in the operational amplifier. A capacitor of this size generally limits the rate at which the input voltage is allowed to change and still produce accurate analog signals. This shortcoming constitutes a particular limitation in digital-to-analog converters intended for high speed multiplication applications. The compensation circuitry also produces a transient problem even in fixed reference (non-multiplying) digital-to-analog converters. More specifically, the settling time of the output signal from the digital-to-analog converter, following changes in the binary input, is indirectly related to the settling time of the reference amplifier, since parasitic coupling paths normally cause small impulse excitation of the reference amplifier during switching transients.
Accordingly, a principal object of the present invention is to provide a current for biasing the ladder of a digital-to-analog converter, or for similar applications, which is strictly proportional to a reference voltage, and which provides this proportionality over a wide temperature range. Further, it is desirable that these properties be obtained while retaining the desirable features of an operational amplifier, including fixing of the voltage level of the supplied current.
Additional objects are to simplify and to reduce the settling time of digital to analog converters.